O'nyong-nyong fever in south-central Uganda, 1996-1997: Description of theepidemic and results of a household-based seroprevalence survey

O'nyong-nyong (ONN) fever, an acute, nonfatal illness characterized by poly arthralgia, is caused by infection with a mosquito-borne central African al phavirus. During 1996-1997, south-central Uganda experienced the second ONN fever epidemic ever recognized. During January and early February 1997, ac tive case-finding and a household cluster serosurvey were conducted in two affected and two comparison areas. A confirmed case was defined as an acute febrile illness with polyarthralgia occurring within the previous 9 months plus serologic confirmation or isolation of ONN virus from blood. In affec ted (n = 129) and comparison (n = 115) areas, the estimated infection rates were 45% and 3%, respectively, and the estimated attack rates were 29% and 0%, respectively, for an apparent:inapparent infection ratio of nearly 2 i n affected areas. In villages sampled near Lake Kijanebalola, Rakai Distric t, the estimated infection and attack rates were 68% and 41%, respectively, and 55% of sampled households had greater than or equal to 1 case of ONN f ever. In conclusion, this epidemic was focused near lakes and swamps, where it was associated with high infection and attack rates.